DEFINITION OF THE CLASS actors might he used in addition to those which will be made use" of in the following brief catalogue of essential mammalian features, were it not for the low-placed Monotremata on the one hand and the highly specialised Whales 011 the other. Including those forms, the Mammalia are to be distinguished from all other Vertebrates by the following series of structural features, which will he expanded later into a short disquisition upon the general structure of the Mammalia. The class Mammalia may, in fact, be thus defined :--- Hair-clad Vertebrates, with cutaneous glands in the female, secreting milk for the nourishment of the young. Skull without prefrontul, post frontal, qitadrato-jugal, and some other bones, and with two occipital condyles formed entirely by the exoccipitals. "Lower jaw composed of dciitary bone only, articulating only with the s<]uamo8al. Kar bones a chain of three or four separate bonelets. Cervical vertebrae sharply distinguished from the dorsals, and if with free ribs, showing no transition bet woo n these and who thoracic ribs. [Bruin, with four optic lobes. .Lungs and heart separated from abdominal cavity by a muwular diaphragm. "Heart with a single left aortic arch. lied blood-curpnF.rlcs non-nuc-loji.tt». The following characters are also very iifarJy inuvor.saj, a,nd in any ca.se absolutely diKtinetivt' :---Otirvioal vort.^brao, seven; vertebrae with epiphytes. Ankle-John '*" c'ruro-tiuvjil," ?.r. be- tween tlie leg and the ankle, and. not in the middle of the ankle.1 Attachment of the pelvis to the vertebral coluiH.fi pre-ucetubitlar in position. The Mammalia >smee they are hot-Hoot!r-d t»rt«iturft« are more independent of temperature than reptik'R ; they ?m>. thtm fouml Hpread over a wider area of tins ca-rthV Huricu*.(\ AK h* .vv-uver, though hot-blooded, they have not tin*. j*ower.H of lowmoti*'*u ]»ott8e«aetl by birds, they are not quite HO widely distiibiiUHl as arc-* thofte animals. The IVfanimiilia nuij.v*. up uit-o tho i.»xtivr^<» luifUi. but, cxcoptiug only forms inauily *,Ujiu»,tu(. such tin the Sea. JJ^Bfci,, are not known to occur on tho .Antarctic, continent'. "Witii tho i's< of the flying Hats, indigcmoiiB tuanirnalH art',* totally alwcm Hew Zealand; and it seem** to be <Ii>ul>tt*ul whether tho#e sup- posed oceanic islands which have a xnajatznttliaiL fUima really 1 Tiic dege»eratlon of tlie hiwd-li?Hb in,Wti»lr» awl Bwtjsi* foi'l«*ls fcl» *»e of tMs cl»»ira€ter w m 4iHtinefcive o»e on tUo pTiaelplefe ?ntvo««,ie<i "by tlut tfelaoti^m of tii« above liat. But it would bo absurd to leaw u»ut lialr.